A glow plug is known which is mounted on an internal combustion engine and in which a sensor for measuring the pressure in a combustion chamber is incorporated in a housing. In an example of a structure for achieving such pressure measurement, a rod-shaped heater is configured to be displaced in an axial direction in response to pressure, and the displacement of the rod-shaped heater is detected. In order to ensure airtightness between the heater and the housing while a displacement of the heater is achieved to a detectable degree in this method, a configuration is known in which the heater and the housing are connected to each other by a variable member (elastic portion). Such a glow plug is configured such that, when the variable member receives a pressure, an axial force is exerted to the rod-shaped heater (e.g., Patent Document 1).